1. Field of the Invention
The invention generally relates to malaria vaccines. In particular, the invention provides malaria vaccines based on polyepitope constructs that elicit cell-mediated immunity against the circumsporozoite protein of a broad spectrum of Plasmodium falciparum parasite species and that are recognized by the majority of HLA alleles.
2. Background of the Invention
Malaria is a vector-borne infectious disease that is widespread in tropical and subtropical regions, including parts of the Americas, Asia, and Africa. Each year, it causes disease in approximately 650 million people and kills between one and three million, most of them young children in Sub-Saharan Africa.
The disease is caused by protozoan parasites of the genus Plasmodium. The most serious forms of the disease are caused by Plasmodium falciparum and Plasmodium vivax, but other related species (Plasmodium ovale, Plasmodium malariae, and sometimes Plasmodium knowlesi) can also infect humans.
Malaria parasites are transmitted by the bite of female Anopheles mosquitoes which delivers the sporozoite form of the parasite to the human host. The sporozoites are carried via the blood to the liver, where they multiply within hepatocytes and evolve to the next form of the life cycle, merozoites. Merozoites are ultimately released into the blood stream when infected hepatocytes rupture. The merozoites then infect red blood cells and some become male and female gametocytes within the RBCs. When another mosquito bites the infected host, it ingests male and female gametocytes, which fuse to become sprozoites within the female mosquito. The sporozoites are then passed to yet another host when the mosquito next feeds, and so on.
Malaria infection causes symptoms characteristic of anemia (light headedness, shortness of breath, tachycardia etc.), as well as other general symptoms such as fever, chills, nausea, flu-like illness, and in severe cases, coma and death. Malaria transmission can be reduced by preventing mosquito bites with mosquito nets and insect repellents, which, although quite effective, have the disadvantage of requiring distribution and proper consistent use. Other measures include mosquito control by spraying insecticides inside houses and draining standing water where mosquitoes lay their eggs. Unfortunately, the use of insecticides poses environmental risks, and in some areas, it is virtually impossible to drain all standing water.
After being naturally infected with Plasmodium, human hosts produce anti-Plasmodium antibodies. However, the ability to neutralize the parasite solely by antibody production (humoral immunity) does not last as habitual, defensive immunity nor is cellular immunity with sufficient memory characteristics elicited. As a result, infection may occur any number of times, complicating disease treatment and prevention.
Several attempts have been made to produce an effective anti-malaria vaccine. U.S. Pat. No. 6,660,498 to Hui et al., describes the use of a baculovirus system to produce recombinant Major Merozoite Surface Protein 1 for use in a vaccine; U.S. Pat. No. 5,393,523 to Knapp et al., describes the preparation and use in a vaccine of recombinant histidine-rich protein of P. falciparum; U.S. Pat. Nos. 4,957,738 and 4,735,799 to Patarroyo describe a mixture of synthetic peptide compounds which induce antibodies against the late stages of P. falciparum malaria; U.S. Pat. No. 4,643,896 to Asakura discloses a novel malaria associated antigen, CRA, that is described as useful as a malaria vaccine.
Unfortunately, in spite of previous efforts, no effective vaccine is currently available against malaria. In particular, no vaccine that elicits cell-mediated immunity against all or most strains of malaria parasites has been developed. Instead, preventative drugs such as quinine or artemisinin derivatives must be taken continuously to reduce the risk of infection. These prophylactic drug treatments are often too expensive for most people living in endemic areas. Further, drug resistant strains of the parasite are increasingly common.
Vaccines encoding or constructed from protein fragments of the circumsporozoite protein (herein CSP) have shown immunogenicity and some limited protective capacity in both humans and animal models. The first successful, although impractical, vaccine against malaria consisted of irradiated mosquitoes carrying sporozoites. Significantly, a subunit vaccine delivered in adjuvant designed to elicit cellular immune responses provided greater protection in humans than either CSP or adjuvant alone. Variable regions of both the carboxy and amino termini have been identified that contain epitopes recognized by humans with a variety of HLA types. The major shortcomings of CSP based vaccines to date have been the breadth of HLA types that bind to the epitopes of any particular CSP sequence and the duration of cellular and humoral immune responses.
The prior art has thus-far failed to provide a safe, effective malaria vaccine, particularly one that effectively elicits a lasting cell mediated immune response to most strains of the parasite.